Keeping Bettas with Other Fish
Many people wonder whether you can successfully keep Bettas with other fish in your aquarium. In short, yes you can! However, there are a few precautions you'll need to take first. Bettas by nature are very territorial. They generally won't back down from a fight, and can be pugnacious. They also have very long, flashy fins which are fairly fragile. They are a tropical fish, and thus require their aquarium to be heated. I've also noticed that Bettas thoroughly enjoy decorations and hiding spots in their aquariums. With all of that being said, you'll need to keep this things in mind when choosing tank mates.
In my own opinion, it is cruel to keep a Betta in anything less than a 5 gallon tank. Just because an animal can survive in cramp conditions doesn't mean you should subject them to that. I feel that a 10 gallon tank is a great size for a Betta that has tank mates, and the absolute minimum.
Using a 10 gallon aquarium ensures that the other fish can get away from the Betta in case there is a chase or bullying. A 10 gallon also makes cleaning and keeping your tank at the correct temperature much easier. I very much recommend to have decorations, plants (fake or live), and areas in which to hide in your tank. This is another way to keep your Betta happy, and the other fish able to get away from the Betta.
The Tank Mates
This is obviously the most important part of keeping your Betta with other fish. I don't recommend keeping fish that are prone to fin nipping (ie, barbs), have long, flashy fins themselves (ie, angelfish), or are aggressive themselves. The ideal tank mate for a Betta is a drab, peaceful fish that is quick enough to get out of the way. Of all of the Betta fish I've kept with other species, I've never had a problem with bullying or having a fish suffer from it. Make sure you do plenty of research on the fish you're interested in. And, after bringing your fish home, make sure you quarantine everyone before putting them together.
Currently I have a 10 gallon tank (lights, heater & filter) that has a male betta, 1 female guppy (the other I bought mysteriously died), and 2 cory cats. Everyone gets along perfectly fine. The little guppy Nessie will go up right next to the Betta Bowie during feeding time. As you can see in the picture above, Bowie will take a nibble from the algae wafers with the Cory cats as well. Occasionally Bowie will chase after Nessie. She easily darts out of the way and they go back to whatever they were doing before. She hasn't suffered at all from being housed with a the "big, bad Betta". I'm planning to move a few more guppies in with Nessie to keep her company. Also, to clarify about guppies. I would be extremely cautious about keeping a Betta with male fancy guppies since he may see them as other Bettas. Nessie is a "feeder" guppy and quite drab. Male feeder guppies, I think, are quite cute and much prettier than fancy guppies, and can be kept with Bettas successfully.